P1,P4-bis(uridyl)tetraphosphate (trivial name: “UP4U”) represented by the formula [I] below or its salts are used as therapeutic agents for keratoconjunctival epithelial disorders due to dry eye. Furthermore, the above compounds have the effect of inducing expectoration and are therefore expected to be developed as expectorants or therapeutic agents for pneumonia.

In order to enable P1,P4-bis(5′-uridyl)tetraphosphate (hereinafter also referred to as “UP4U”) to be stably stored, crystals of this compound or a salt thereof are obtained (see Patent Literature 1). However, the conventional literature states that the crystals are stable as compared with a lyophilized product, but does not consider under what condition the crystals can be stably stored.